This invention relates generally to digital transceivers, and more particularly to transceiving adaptively pre-coded digital signals in an asymmetric communications network.
Recently, broadband fixed wireless access technology has been widely regarded as an effective way to realize point-to-multipoint high data rate transmissions, as evidenced by standardization activities in ETSI BRAN and IEEE802.16, see RES 10/96 ETR7- 10C Report, xe2x80x9cThe ETSI HIPERLAN Layer Architecture,xe2x80x9d October 1996, and the IEEE802.16 Fixed Wireless Access Standard Committee draft for IEEE802.16 FWA, under development.
Broadband wireless systems for multimedia applications must combat fading, multipath impairments, and inter-symbol interference (ISI) that can degrade the received signal. The very high data rates, e.g., 20 Mb/s or higher, at a low bit error rate for multimedia data, together with high carrier frequencies, ranging from time-dispersive Microwave Multipoint Distribution System (MMDS) bands ( less than 10 GHz) to fading dominated millimeter-wave bands (e.g. 10-66 GHz), make these issues particularly difficult to solve.
A number of approaches have been proposed in recent years, see Matsumoto et al. xe2x80x9cConfiguration and performance of transmission-diversity-combined OFDM systems for broadband wireless access,xe2x80x9d Globecom2000, and Kang et al. xe2x80x9cModulo-Type Viterbi Decoder for High-Speed Digital Transmission Systems,xe2x80x9d IEEE Comm. Let., Vol. 4, No. 2, February 2000.
These prior art approaches include single carrier M-ary quadrature amplitude modulation (M-QAM) systems and multicarrier systems based on Orthogonal Frequency Division Multiplexing (OFDM). They differ in performance as well as implementation complexities.
Of particular importance is a fixed wireless access system. For the purpose of this description, a base station (xe2x80x9ccell towerxe2x80x9d) of the wireless system will be referred to as the xe2x80x9ctransmitter,xe2x80x9d and the client radio, e.g., cellular telephone, as the xe2x80x9creceiver.xe2x80x9d It should be understood that both the base station and client can transmit and receive signals.
One characteristic of such a fixed wireless system is its asymmetric nature. The system has a large downlink from the base station (transmitter) to many clients (receivers). The system has a smaller uplink from each individual client xe2x80x9creceiverxe2x80x9d to the base station xe2x80x9ctransmitter.xe2x80x9d The relatively high installation cost of installing and operating a small number of base stations, compared to a multitude of relatively low-cost client radios, lead one to pursue system designs that minimize the complexity, power consumption and cost for the clients, by adding more processing to base stations.
One prior art solution to reduce signal degradation uses large decision feedback equalizers in the receiver. However, this solution increases the computational complexity and cost of the receiver. The situation worsens as multipath distortion increases. Often, long training sequences must be sent along with the (information) data to help the receiver""s equalizers to converge. This decreases the amount of information that can be transmitted in a specific amount of time.
Therefore, it is desired to provide a transceiver system with low cost and low complexity receivers that have reduced signal degradation.
The invention provides a radio transceiver system that reduces signal degradation without unduly increasing complexity and cost of the receiver. The transceiver system according to the invention is based on adaptive channel pre-coding. In the present transceiver system, a transmitter applies dynamically adapted pre-coding to the input data, prior to transmission. The adaptation of the pre-coding is based on feedback received from the receiver.
The feedback allows the transmitter to determine the average tap energy of the receiver""s equalizers, and the peak-to-average ratio. This results in reliable operation over time-varying channel conditions, making the invention suitable for wireless and wireline systems with large interference. Furthermore, the adaptive pre-coding transceiver system according to the invention reduces the complexity of the receiver. The present transceiver system is capable of delivering multimedia voice, data, and video with high fidelity.
The transceiver system uses an adaptive Tomlinson-Harashima (TH) pre-coder and a modulo-type Viterbi decoder. The transceiver combines spatial diversity with adaptive modulation. These features result in the following advantages.
Error propagation problems, as would be experienced by conventional decision feedback equalizers (DFE), are minimized, and ISI are compensated. Equalization gain and full coding gain of trellis code are achieved. The system complexity of the receiver is significantly reduced. Spatial diversity is achieved by using transmission diversity and an adaptive antenna system, which improve throughputs and multipath interference rejection. Adaptive modulation increases spectral efficiency and service coverage by allowing different levels of services to users.
More particularly, the invention provides a method and apparatus for adaptively pre-coding signals transmitted in a communications network. A digital transceiver system includes a downlink channel and an uplink channel. In the system, a transmitter transmits signals on the downlink channel and receives signals on the uplink channel.
The transmitter includes a pre-coder having a plurality of filters. Each filter has a corresponding filter coefficient. The transmitter also includes a filter coefficient updater coupled to the pre-coder. A receiver of the system receives signals on the downlink channel and transmits signals on the uplink channel. The receiver includes linear equalizers having a plurality of taps. Each tap has a corresponding tap coefficient. The coefficient updater is configured to dynamically update the filter coefficients of the pre-coder according to the tap coefficients which are indicative of the frequency response of the receiver.